Semiconductor switching device and method

ABSTRACT

A semiconductor switching device ( 10 ) is formed on a semiconductor substrate ( 12 ) having a trench ( 44 ) formed on one of its surfaces ( 42 ). A control electrode ( 32 ) activates a wall of the trench to form a conduction channel ( 36 ). A first conduction electrode ( 40 ) is disposed on the semiconductor substrate to have a first doped region ( 34 ) for receiving a current and a second doped region ( 24 ) for routing the current to the conduction channel.

FIELD OF THE INVENTION

[0001] The present invention relates in general to semiconductor devicesand, more particularly, to high current, high voltage bidirectionalswitching devices.

BACKGROUND OF THE INVENTION

[0002] Bidirectional switches are power devices that switch highcurrents through their conduction electrodes while blocking highvoltages applied to the conduction electrodes. Bidirectional switchesare used in a variety of electrical systems. For example, batterychargers often use a bidirectional switch to control a current flowingbetween a terminal of the charger to a terminal of a battery beingcharged. A control voltage is applied to a control electrode of thebidirectional switch to close the switch and allow current to flow fromthe charger terminal to the battery terminal to charge the battery. Whenthe control voltage is removed, the bidirectional switch opens and nocurrent flows. If the charger is turned off while the battery isattached, the switch blocks the voltage on the battery terminal, and ifthe battery is removed while the charger is turned on, the switch blocksthe voltage on the charger terminal. A typical bidirectional switch isspecified to supply at least one ampere of current while blockingvoltages of at least eight volts without breaking down.

[0003] Standard planar power metal-oxide-semiconductor field effecttransistors (MOSFET) are not adequate for use as bidirectional switchesbecause they have low gate to source breakdowns, often as low as fivevolts, and have a low current capability due to low packing density fora given breakdown voltage. Standard vertical power MOSFETs also are notadequate because of a low breakdown in one direction, making themunidirectional devices. Consequently, current bidirectional switchestypically are implemented using two separate serially coupled powerMOSFETs, with the drain of each transistor operating as a conductionelectrode of the switch. The separate MOSFETs are formed on separatesemiconductor dice and housed in separate packages, which results in ahigh manufacturing cost and a large area occupied on a circuit board.When the separate MOSFET dice are housed in a single package andinterconnected with wire bonds, the area occupied on a circuit board isreduced but the manufacturing cost is still too high for manyapplications.

[0004] Hence, it would be a benefit if there were a bidirectional switchthat was manufactured on a single semiconductor die in order to reducethe manufacturing cost of the bidirectional switch.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a schematic diagram of a battery charger charging abattery;

[0006]FIG. 2 shows a cross-sectional view of a bidirectional switch in afirst embodiment;

[0007]FIG. 3 shows a cross-sectional view of a bidirectional switch in asecond embodiment;

[0008]FIG. 4 shows a cross-sectional view of a bidirectional switch in athird embodiment;

[0009]FIG. 5 shows a cross-sectional view of a bidirectional switch in afourth embodiment;

[0010]FIG. 6 shows a cross-sectional view of a bidirectional switch in afifth embodiment; and

[0011]FIG. 7 shows a cross-sectional view of a bidirectional switch in asixth embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

[0012] In the figures, elements having the same reference numbers havesimilar functionality.

[0013]FIG. 1 is a schematic diagram of a battery charger 2 for charginga battery 8. Battery charger 2 includes a control circuit 4, a chargingcircuit 6 and a bidirectional switch 10. In one embodiment, battery 8 isconfigured as an eight volt battery.

[0014] Control circuit 4 has a sense input 5 for monitoring a voltageV_(D) across battery 8. Voltage V_(D) is processed and the charge statusis provided on a status output coupled to a node 7 and received bycharging circuit 6 for generating an appropriate charging voltageV_(s)=8.0. A control output applies a voltage V_(G)=3.0 volts to acontrol or gate lead 30 of bidirectional switch 10, which routes acharging current from charging circuit 6 to battery 8 during normaloperation. If a fault condition such as a shorted battery or chargingmalfunction occurs, control circuit 4 sets V_(G) to zero volts to turnoff bidirectional switch 10 to isolate charging circuit 6 from battery8.

[0015] Bidirectional switch 10 has first and second current carryingpackage leads designated as a source lead 28 and a drain lead 16, whichoperate at voltages V_(s) and V_(D), respectively. When V_(G) is zerovolts, voltage on both source lead 28 and drain lead 16 is blocked,thereby avoiding a current flow from either charging circuit 6 orbattery 8 that could result in damage. In one embodiment, bidirectionalswitch 10 is specified to operate with a current flow of at least oneampere when turned on, and to block a voltage of at least eight voltsapplied between drain lead 16 and source lead 28 or between gate lead 30and either source lead 28 or drain lead 16 when turned off.

[0016]FIG. 2 is a cross-sectional view of bidirectional switch 10 formedin a semiconductor substrate 12 for switching a current between drainlead 16 and a source lead 28 in response to voltage V_(G) applied togate lead 30.

[0017] Semiconductor substrate 12 has a first surface 42 which isselectively etched to a depth of about 2.7 micrometers to form a trench44. Semiconductor substrate 12 has a second surface 46 for mounting to adie attach flag 14 of a semiconductor device package. Die attach flag 14is formed with a metal or other highly conductive material and coupledto drain lead 16 to provide a low resistance current path. Semiconductorsubstrate 12 is layered by successive doping steps to produce dopedregions designated as a drain 18, a drain extension 20, a body region 22and a source extension 24.

[0018] Drain 18 and drain extension 20 cooperate to function as a drainelectrode 48. Drain 18 is formed adjacent to second surface 46 and isheavily doped with donor atoms to produce a low resistivity region ofn-type conductivity and to provide an ohmic contact to die attach flag14.

[0019] Drain extension 20 extends upward from drain 18 to a depth ofabout 2.2 micrometers from first surface 42, which is about 0.5micrometers above the bottom of trench 44. Drain extension 20 is dopedwith donor atoms to produce a material with n-type conductivity but witha lower doping concentration than drain 18. The thickness of drainextension 20 is selected to provide a blocking or breakdown voltage fromdrain lead 16 to either source lead 28 or gate lead 30 of at least eightvolts as described below. In one embodiment, drain extension 20 has athickness of about two micrometers and a doping concentration of about5*10¹⁵ atoms/centimeter³.

[0020] Body region 22 is lightly doped with acceptor atoms to produce amaterial with p-type conductivity which can be inverted to activate aconduction channel 36 when a control voltage exceeds a conductionthreshold of bidirectional switch 10. In one embodiment, a gate lead 30to source lead 28 conduction threshold is 1.5 volts and a gate lead 30to drain lead 16 conduction threshold is 1.5 volts. Body region 22 has athickness of about 1.2 micrometers and a doping concentration of about5*10¹⁵ atoms/centimeter³.

[0021] Source extension 24 is formed adjacent to first surface 42 and isdoped with donor atoms to produce a material with n-type conductivity.The thickness and doping concentration are selected to provide a desiredblocking or breakdown voltage greater than eight volts from source lead28 to either drain lead 16 or gate lead 30. In one embodiment, sourceextension region is formed with a thickness of about 0.9 micrometers anda doping concentration of about 1.8*10¹⁷ atoms/centimeter³. Note thatthe embodiment of FIG. 1 shows separate regions designated as sourceextension 24. Depending on the specified current capability,bidirectional switch 10 may include a multiplicity of such commonlydesignated regions, which are coupled together out of the view plane ofFIG. 1 to function as a single region. Such features are not shown inorder to more clearly show and describe the invention.

[0022] Source 34 is formed adjacent to source extension 24 byselectively introducing donor atoms from surface 42 to a typical depthof about 0.4 micrometers to provide n-type conductivity. The dopingconcentration is high in order to provide an ohmic contact from source34 to source lead 28. In one embodiment, source 34 has a dopingconcentration of 3.0*10¹⁹ atoms/centimeter³.

[0023] A dielectric layer 26 includes a region 52 overlying firstsurface 42, a region 54 adjacent to walls of trench 44, and a spacer 27adjacent to a gate electrode 32. In one embodiment, dielectric layer 26comprises silicon dioxide. The thickness of dielectric layer 26 inregions 52 and 54 is similar, and consequently such regions typicallyare formed during the same processing step. Dielectric layer 26 is madesufficiently thin in region 54 to function as a gate dielectric forbidirectional switch 10. Hence, if a source voltage V_(s) operating atground potential is applied to source lead 30 while a positive gatevoltage V_(G) greater than the conduction threshold is applied to gatelead 30, body region 22 is inverted adjacent to region 54 to formconduction channel 36 between source extension 24 and drain extension20. Similarly, if drain lead 16 operates at a drain voltage V_(D)=0.0volts while a positive voltage V_(G) greater than the conductionthreshold is applied to gate lead 30, body region 22 inverts to formconduction channel 36 adjacent to region 54. In one embodiment, regions52 and 54 have a thickness of about two hundred angstroms, so a voltagegreater than a critical voltage of five volts across regions 52 and/or54 may result in an undesirable breakdown of dielectric layer 26. If notavoided, such a breakdown would allow current to flow from gate lead 30to either drain lead 16 or source lead 28 and could result in permanentdevice damage.

[0024] Gate electrode 32 is formed adjacent to dielectric region 54 andextends above first surface 42 to overlie dielectric region 52. Gateelectrode 32 typically comprises doped polysilicon to provide a lowinterconnect resistance and an ohmic contact to gate lead 30.

[0025] Spacer 27 is used to electrically isolate gate electrode 32 fromsource 34 after metallization. Spacer 27 further cooperates with gateelectrode 32 during the formation of source 34 to function as a mask toblock the introduction of impurities, thereby defining a distance D2from gate electrode 32 to an edge of source 34. In one embodiment,spacer 27 is formed so that distance D2 is at least 0.4 micrometers.

[0026] The operation of bidirectional switch 10 proceeds as follows.Assume source lead 28 is biased at voltage V_(s)=0.0 volts, gate lead 30is biased at voltage V_(G)=3.0 volts and drain lead 16 is biased atvoltage V_(D)=8.0 volts. Since the gate to source voltage (V_(G)-V_(S))is greater than the conduction threshold of 1.5 volts, body region 22 isinverted to form conduction channel 36. A current flows from source lead28 to source 34 and is successively routed through source extension 24,conduction channel 36, drain extension 20 and drain contact region 14 todie attach flag 14 and drain lead 16. In short, the current flows fromsource lead 28 to drain lead 16. To turn off bidirectional switch 10,gate voltage V_(G) is reduced below the conduction threshold, e.g., toground potential, so no conduction channel is formed and no currentflows.

[0027] Similarly, when source lead 28 is biased at voltage V_(s)=8.0volts, gate lead 30 is biased at voltage V_(G)=3.0 volts and drain lead16 is biased at voltage V_(D)=0.0 volts, body region 22 is inverted toform conduction channel 36. A current flows from drain lead 16 to drain18 which is successively routed through drain extension 20, conductionchannel 36, source extension 24 and source contact region 42 to sourcelead 28. To turn off bidirectional switch 10, gate voltage V_(G) isreduced below the conduction threshold, e.g., to ground potential, so noconduction channel is formed and no current flows.

[0028] When bidirectional switch 10 is turned off, high voltages eitherfrom drain lead 16 to source lead 28 or from source lead 28 to drainlead 16 are blocked because the source-body diode is effectivelyconnected back-to-back with the drain-body diode. The voltage fromsource lead 28 to gate lead 30 produces a depletion region 35 in sourceextension 24 adjacent to dielectric regions 52 and 54 as shown. Becausesource extension region 24 has a lower doping concentration than source34, source extension region 24 operates as a drift region that causes adrop of the electric potential across the source extension 24, therebyreducing the voltage across dielectric region 54 to below the breakdownlevel. Hence, no breakdown occurs. Note in one embodiment, source 34 isformed so that distance D2 from gate electrode 32 is at least 0.4micrometers, which is sufficient to avoid a breakdown of dielectriclayer 26 when eight volts is applied to source electrode 28.

[0029] Similarly, when bidirectional switch 10 is turned off, thevoltage from drain lead 16 to gate lead 30 causes a depletion region 37to form in drain extension 20 in the vicinity of dielectric region 54.Drain extension 20 has a lower doping concentration than drain 18, andtherefore operates as a drift region that causes a drop of the electricpotential across drain extension 20. This voltage drop effectivelyreduces the voltage across dielectric region 54 to below the breakdownlevel. Hence, no breakdown occurs. To ensure a sufficient voltage drop,a distance D1 from gate electrode 32 to the bottom of drain extension 20is made larger than depletion region 37. In one embodiment, distance D1is approximately 1.5 um to ensure that eight volts applied to drainelectrode 16 is effectively blocked and dielectric layer 26 does notbreak down.

[0030] Therefore, bidirectional switch 10 blocks high voltages in bothdirections, i.e., either on source lead 28 or on drain lead 16. Blockingoccurs because source and drain extensions 24 and 20 are formed withdoping concentrations and widths sufficient to prevent depletion regionsfrom extending to source 34 and/or drain 18, respectively.

[0031] Note that the vertical structure of bidirectional switch 10,where current flows vertically through conduction channel 36, allowsbidirectional switch 10 to occupy a smaller area of semiconductorsubstrate 12 in comparison to planar devices.

[0032]FIG. 3 shows bidirectional switch 10 in a second embodiment.Elements described in FIG. 1 have similar structures and operationexcept that the second embodiment has the advantage that it is notnecessary to form a spacer adjacent to gate electrode 32 to maintain aseparation from source 34. Instead, region 52 of dielectric layer 26 isformed to a greater thickness than the thickness in region 54. Theincreased thickness reduces the electric field across region 52 to avalue less than a critical breakdown field and maintains distance D2=0.4micrometers between source 34 and gate electrode 32. In one embodiment,the thickness of dielectric layer 26 in region 52 is four thousandangstroms.

[0033]FIG. 4 shows bidirectional switch 10 in a third embodiment.Elements previously described have similar structures and functionalityexcept that source 34 is replaced by a source contact layer 64 which isformed to a similar thickness and doping concentration to provide ohmiccontact to source lead 28. Source contact layer 64 is formed as ablanket deposition by either diffusion or by an epitaxial growthoverlying source extension 24. In one embodiment, source contact layer64 is formed prior to, and defined by, trench 44. Hence, formation doesnot require a separate photolithography step.

[0034] Gate electrode 32 is formed within trench 44 and below surface 42a distance adequate to space gate electrode 32 at least 0.4 micrometersfrom source contact layer 64, thereby avoiding a breakdown at less thanthe specified voltage. A gate conductor 66 comprising a metal such asaluminum overlies gate electrode 32, with which it has an ohmic contactto reduce resistance. Gate conductor 66 is defined by an insulativespacer 67 to maintain distance D2 at least 0.4 micrometers between gateconductor 66 and source contact layer 64 to prevent a breakdown at lessthan the specified voltage.

[0035]FIG. 5 shows bidirectional switch 10 in a fourth embodiment. Thestructure and operation are similar to the description of the embodimentdescribed in FIG. 3, except that no gate conductor is formed over gateelectrode 32. Instead, connection from gate electrode 32 to gate lead 30(not shown in FIG. 4) is made locally at one or more other locations onthe die which are not shown. Gate electrode 32 is formed to be recessedbelow the level of surface 42 as shown by an amount that ensures anadequate spacing to source contact layer 64 to avoid a low breakdownvoltage. Dielectric region 71 is formed over gate electrode 32 andpatterned to expose source contact layer 64 for connecting to sourcelead 28. The embodiment of FIG. 4 has an advantage that source contactlayer 64 is formed as a blanket layer whose edge is defined by trench44, thereby eliminating an additional photomasking step.

[0036]FIG. 6 shows bidirectional switch 10 in a fifth embodiment. Thestructure and operation are similar to the description of the embodimentdescribed in FIG. 1, except that gate electrode 32 does not extend tooverlie surface 42. An interlayer dielectric 69 is deposited onsubstrate 12 and patterned to overlie gate electrode 32 as shown.Interlayer dielectric 69 functions as a mask during the introduction ofimpurities to form source 34, and thereby defines the distance fromsource 34 to gate electrode 32. This distance is at least 0.4micrometers to prevent a breakdown at the specified voltage. Contact ismade to gate electrode using vias (not shown).

[0037]FIG. 7 shows bidirectional switch 10 in a sixth embodiment. Thestructure and operation are similar to the description of the embodimentdescribed in FIG. 1, except that drain extension 20 is replaced with aburied drain extension 70 formed with a buried layer which updiffusesinto body region 22 during an epitaxial processing step to a levelapproximately 0.5 micrometers above the bottom of trench 44.Alternatively, dopants can be introduced at the bottom of trench 44which diffuse down to drain 18 to form a drain extension. The dopingconcentration and thickness are similar to those described in FIG. 1 fordrain extension 20.

[0038] By now it should be appreciated that the present inventionprovides a bidirectional switch that can support a high current flowwhile blocking high voltages applied to either conduction electrode. Asemiconductor substrate has a surface in which a trench is formed. Agate activates a wall of the trench to form a conduction channel, and aconduction electrode has a first doped region for receiving a currentand a second doped region for routing the current to the conductionchannel. The bidirectional switch is fabricated using standardsemiconductor processing and therefore has a low manufacturing cost.Moreover, the bidirectional switch is fabricated on a singlesemiconductor die to further reduce the cost of manufacturing and usingthe device. The vertical structure of the bidirectional switch has afurther advantage of occupying a smaller die area than a planar deviceproviding similar capabilities. It is understood that the presentinvention can be of benefit when used in embodiments not specificallydescribed herein. For example, a p-channel bidirectional switch can bemade by reversing the dopant types described above.

1-11. (canceled)
 12. A bidirectional switch, comprising: a semiconductorsubstrate having a first surface for forming a trench; a controlstructure formed in the trench to enable a conduction channel inresponse to a control signal, and having a dielectric layer along a wallof the trench and a control electrode adjacent to the dielectric layer;a first conduction electrode having a first layer formed at the firstsurface for receiving a current and a second layer underlying the firstlayer for coupling the current to the conduction channel, wherein thefirst layer is spaced a predefined distance from the control structure;and a second conduction electrode disposed at a lower portion of thetrench for receiving the current from the conduction channel.
 13. Thebidirectional switch of claim 11, wherein the second conductionelectrode extends from the lower portion of the trench to a secondsurface of the semiconductor substrate.
 14. The bidirectional switch ofclaim 11, wherein a breakdown voltage between the control electrode andthe first conduction electrode is greater than eight volts.
 15. Thebidirectional switch of claim 13, wherein a breakdown voltage betweenthe control electrode and the second conduction electrode is greaterthan eight volts. 16-20. (canceled)
 21. The bidirectional switch of theclaim 12 wherein the dielectric layer extends onto the first surface,and wherein the dielectric layer has a first thickness along the walland a second thickness along the first surface, wherein the secondthickness is greater than the first thickness.
 22. The bidirectionalswitch of claim 12 wherein the control structure further includesinsulative spacers formed within the trench to provide the predefineddistance.
 23. The bidirectional switch of claim 22 further comprising agate conductor formed in an opening defined by the insulative spacersand coupled to the control electrode.
 24. The bidirectional switch ofclaim 12 wherein the predefined distance is greater than approximately0.4 micrometers.
 25. The bidirectional switch of claim 12 wherein thesecond conduction electrode comprises a drain extension region overlyinga drain region.
 26. The bidirectional switch of claim 25 wherein thedrain extension region comprises a buried drain extension region. 27.The bidirectional switch of claim 12 wherein the first layer is ablanket layer.